Process for producing cold-rolled steel sheets to be deep drawn



Oct. 4, 1966 KAMEO MATSUKURA ETA!- 3,

PROCESS FOR PRODUCING COLD-ROLLED STEEL SHEETS TO BE DEEP DRAWN FiledNov. 20, 1962 2 Sheets-Sheet 1 FIG. 1A FIG. 1 B

Normalized sheet Hot-rolled sheet 7.2 of grain size No. 9.6 of gram sizeNo as left to cool In air after being heated at 950 C Kameo MatsukuraYasuo Shinagawa C 1966 KAMEO MATSUKURA ETAL 3,276,917

PROCESS FOR PRODUCING COLD-ROLLED STEEL SHEETS TO BE DEEP DRAWN 2Sheets-Sheet 2 Filed .NOV. 20, 1962 FIG. 2A

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30 4O 5O 7O Cold-rolling rate in INVENTORS KAMEO MATSUKURA YASUOSHINAGAWA BY M UM ATTORN United States Patent Japan Filed Nov. 20, 1962,Ser. No. 238,896 Claims priority, application Japan, Dec. 2, 196136/43,880 3 Claims. (Cl. 148-124) This invention relates to a processforproducing low carbon rimmed cold-rolled steel sheets to becold-pressshaped for use in automobile bodies.

In the conventional process for producing the abovementioned steelsheets a slab made of ingot is hot-rolled into a thin strip with acontinuous rolling mill after soaked in a heating furnace, said strip isthen Water-cooled and coiled, is air-cooled and continuouslyacid-washed, is rolled with a continuous or reversible cold-rolling milland is bright-annealed to be recrystallized so that any hard fibrousstructure produced during the cold-rolling may be eliminated andcold-drawability may be improved. Further, the annealing forrecrystallization is followed by a skin pass and a leveling, in order toprevent stretcher strains from being produced by the presence of yieldpoint elongation in adjusting the shape of the steel sheet and incarrying out the pressing operation.

In the above mentioned conventional process, it is, however, verydifiicult to obtain desirable optimum grain size and deep-drawability,though the steel sheet is kept for a considerably long time below the Atransformation point in the annealing for recrystallization.

As regards a crystal grain size which is determined by the hot-rollingtemperatures and contents of carbon, manganese and other accompanyingelements contained in the steel, it is generally well known fromexperiences in press factories that the size number of Nos. 6.5 to 8.0according to the standard classification of grain size issued by theA.S.T.M. are most suitable to a pressing work. (For instance, the grainsize number of No. 7 has crystal grains amounting to 1024 in across-section of 1 mm. and an average diameter of 0.0355 mm.)

When the number is smaller than No. 6 (the crystal grains will becomelarger) the grains will be so coarse that the surface becomes roughafter the press-shaping and presents a so-called orange peel skin state,thereby the product will reduce so much in its commercial value as not.to be endurable to uses.

011 the other hand, if the numberis larger than No. 8.0, the hardnessand yield strength will become higher, the elongation and Erichsen valuewill reduce in general, the amount of the spring back will become.larger and the product will not be adapted to a deep-drawing work.

If an aluminum-killed steel deoxidized with aluminum, is used in thiscase a considerable degree of deep draw ability will be, of course,obtained with the same crystal grain size and the product will benonageing on account of the production of so-called elongated grainsbased on the difference in the recrystal grain growing velocitydepending on the precipitation of aluminum nitride in the annealing forrecrystallization. However, because of such inherent defects of thealuminumekilled steel as the low yield, poor surface state and highprice, it is not so much used today except for specific uses.

Patented Oct. 4, 1966 Further, it is also very difficult to obtainaluminumkilled steel sheet having regularly optimum grain size favorableto a pressing work and enduring against severe pressing Work.

There are numerous criteria for judging the pressshapability ofcold-rolled steel sheets, but none of them has shown a perfectcorreltion with the rates of poor results.

The Lankford R; value recently proposed by Lankford and others is saidto be one of the best criteria for judging press-shapability, whichvalue is shown by the ratio of the strain in the width direction to thestrain in the thickness direction after carrying out a tension workwithin the plasticity range of uniform elongation (usually an elongationof 20% is adopted) in a-tension test.

This value is known to have a close relationship with the preferredcrystallographic orientation and to show a large number when the grainsare arranged in a certain orientation. According to experiments, the R;value of Lankford and the deep-drawability are closely correlatedparticularly in drawing and composite shaping of drawing and stretching.

In general, the Lankiford R; value will become larger with the increaseof the cold-rolling rate up to some value. But, the increase of thecold-rolling rate will be accompanied by the diminution of crystalgrains on the other hand. That is to say, the yield strength, hardnesswill rise with it. Further, the yield point elongation will increase,producing unfavorable result in stretch forming works. Therefore, inorder to improve the press-shapability of a cold-rolled sheet, it isnecessary to obtain properly large crystal grains while elevating the R;value of Lankford showing a plastic anisotropy.

An object of the present invention is to provide a lowcarbon thin steelsheet having a high deep-drawability by applying a proper heat-treatmentand cold-rolling work to a hot-rolled material sheet, so that the grainsize of the final product may be adapted to a pressing Work, the rate ofelongation, Erichsen value and R; value may be improved, a severe deepdrawing work may be feasible even in the case of using a rimmed materialand such defect as surface roughness may be eliminated.

The invention will now be explained with reference to the accompanyingdrawings in which:

FIG. 1A is a microphotograph showing the structure of hot rolled steelto be treated according to the present invention; I

FIG. 1B is a microphotograph of the structure of the.

steel of FIG. 1A; however, it has been'normalized above the Atransformation point; and

FIGS. 2A-2E are graphs showing various test values for the steel treatedaccording to the present invention.

The object of the present invention is attained by slabbing andhot-rolling an ingot as in an ordinary process so as to make it ahot-rolled coil, normalizing the coil at a temperature above the A pointwith or without prior cold-rolling it at a proper reduction rate,preferably below 50%, then rapidly cooling it, for example by quenchingit, thereafter cold-rolling it at a reduction rate of about 40 to andthen softening (annealing) for recrystallization.

According to the present invention, cold-rolled steel sheet having aproper crystal grain size, endurable to a severe deep-drawing work andhaving a high ductility can be obtained.

In the case of using a rimmed steel as the raw material in the presentinvention, nonageing cold-rolled steel sheets having a crystal grainsize adapted to a pressing work, endurable to a severe deep-drawing workand having a high ductility can be obtained by positively decarburizingand denitriding the material by carrying out the softening annealing inan atmosphere containing wet hydrogen.

The microscopic structure of the hot-rolled coil to be treated accordingto the present invention generally corresponds to Nos. 9.511.0 in thestandard classification of the A.S.T.M. (see FIGURE 1A). However, if theferrite crystal grain coordination has taken place .by normalizing thecoil above the A transformation point according to the presentinvention, grain size will become Nos. 7.0 to 7.5 in the A.S.T.M.classification as shown in FIG- URE 1B. (The chemical composition of thesample was then 0.09% C, 0.39% Mn 0.007% Si, 0.018% P, 0.017% S and0.08% Cu.) In such case, it may be lightly coldrolled before beingnormalized. The cold-rolling rate in such case is determined by therelation between the thickness of the hot-rolled sheet and that of theproduct. The degree of growth of the crystal grains can be freelyadjusted by varying the cold reduction rate, and the time during whichthe normalizing is carried out. Thus, when the hot-rolled material sheetis normalized in a temperature above the A; transformation point withoutor after being lightly cold-rolled, is rapidly cooled, for example bybeing quenched and is then cold-rolled and annealed, the crystal grainsof the final product can be made to be adaptable to the work and therate of elongation, Erich-' sen valueand R| value can be improved.

In the process of the present invention, the hot-rolled coil isnormalized after being lightly cold-rolled depending on thecircumstance,'is quenched and is then coldrolled at a reduction rate ofabout 40 to 80%. In the range of this reduction rate, the crystal grainsize of the obtained cold-rolled steel sheet will be around No. 7.5,i.e. most adapted to the pressing work, wherein the elongation, Erichsenvalue and conical cup value (Fukui CCV) Will be critically improved andthe RL value of Lankford will be also increased. This fact is shown inthe comparative experiment diagrams shown in FIGS. 2A*E. These figuresrepresent test values on the workability of a cold-rolled steel sheetsproduced by the conventional process and that of a cold-rolled steelsheet obtained according to the present invention. As evident from thesediagrams, it is a peculiar phenomenon nnknown in the cold-rolled steelsheet by the conventional process. that the rate of elongation increases.with the rise of the cold reduction rate. I

. As shown by these comparative test values, it is evident that acold-rolled steel sheet suitable to the deep-drawing maybe obtained bynormalizing the hot-rolled material in a temperature range above the Apoint to adjust the grain, after lightly cold-rolled depending on thecircumstance- An example of the process for producing cold-rolled steelsheets for deep-drawing according to the present invention is asfollows:

(1) A rimmed steel slab is hot-rolled and is wound up i in the form of acoil and the coil is cooled an is then coldrolled at a reduction ratebelow 50% (2) Normalizing of the coil for a short time at a temperatureabove the A transformation point (in a temperature range of 10 to 100 C.above the A transformation point) and subsequent rapid cooling, forexample by quenching.

(3) Acid pickling of the normalized and rapidly cooled coil.

(4) Cold-rolling at a reduction rate of 40 to 80%.

(5) Bright-annealing or decarburizing and denitriding annealing forrecrystallization in an atmospheric gas containing wet hydrogen.

(6) Temper rolling.

(7) Leveling.

As the normalizing operation among the above mentioned steps is acomparatively quick quenching, it is, no doubt, desirable to be carriedout in a continuous furnace, but is not always limited to be carried outin the continuous furnace. Even a batch furnace may also be used, if aquick quenching system, for example open coil system, will be adopted.

The atmosphere, in which the normalizing operation is carried out, isnot limited to be of an inert gas. A molten salt, molten metal=oroxidizing gas may also be used. For the rapid cooling after thenormalization, either air-cooled, molten saltor water may be used.

In the above mentioned steps, it is also possible to proceed from thefirst step, in which the strip is wound up at the temperature below 650C., to the second step, without leaving it to cool down to the roomtemperature, but directly feeding it into a furnace, which is kept at ahigh temperature.

An electric cleaning step can be added between the fourth and fifthsteps depending on the circumstance. All the a'bovementioned steps arecarried out with the, strip in the form of a coil, but it is, of course,possible to use the material in theform of cut sheets for any of thelatter half steps. 1

In the annealing for recrystallization, a batch type furnace or acontinuous annealing furnace may be used. The annealing usually carriedout is a bright-annealing in a protective atmosphere,- but is not alwayslimited to that.

It can be carried out also in an oxidizing or reducing atmosphere. It isobvious from the nature of the present invention that decarburizinganddenitriding actions may :be included in either case.

It is another feature of the present invention that a nonageing very lowcarbon steel sheet can be obtained by carrying out decarburization anddenitrification by using an atmosphere containing wet hydrogen innormalizing annealing. In such case, a nonageing steel sheet can be madeof a rimmed steel and the so-called deep-drawability will furtherimprove to be above the values shown in FIGS. 2A-2E. Such extremely lowcarbon steelsheet can be also used for porcelain enameling.

The present invention shall be explained with reference to a concreteexample.

Example.-A hot-rolled sheet having a composition shown in Table 1 and\made in the ordinary steps Otf steel making, slabbing and hot-rollingin a factory was rolled at :a reduction rate of up to 68.5% with alaboratory rolling mill. This steel sheet was normalized (air-cooled) at950 C. for 2 hoursand was then rolled again by 30 to 80%. Thiscold-rolled sheet was annealed at 650 C. for 2 hours in an annealingatmosphere gas called DX (5.0% CO 10.5%' CO, 0.5% CH 12.5% H 0.8% H 0,the rest being N and of a dew point of 5 C.) and was subjected tofurnace cooling.

The deep-drawability of the thus obtained product and that of a productobtained directly by ordinarily annealing the material (in a DX gasatmosphere) at 650 C. for 2 hours without primary cold rolling andnormalizing but ground by surface grinding both surfaces of hot coilTABLE 1.COMPOSITION IN PERCENT OF THE HOT ROLLED MATERIAL SHEET 0 ISi MnP18 Cu 0.09 t 0.007 0.39 i 0.018 I 0.017

TABLE 2.DEEP-DRAWABILITY [Cold-rolled rate in percent] Primarycold-reduction 68.5 53.5 55.5 44.5 27.0 12.5 Drawability Secondary coldreduction 4s. 7 45. (45. 7 47. 0 45. 7 45. 7 Elwgamn 111 Percent E50. 2(51. (52. a 53. a 54. 2 53. 0 10. 45 10. a 10. 9. 95 9. 75 9. 05Erlchsen Value mm i (11.95) (11.90) (11. 85) (11. 8 J 5. 15 5. 05 5. 105. 55 Value mm "i 55 g5 35 4 5. 30) 5. 10 4 5. 72 5 7 1.13 1. 07 0. 55Rankford value 0.85 1.07 1.18 1.47 1.45 0.85 A.S.T.M. crystal grain sizeNo g; g g: 8? $1? 3; g? 3; 3; 3

NOTE-The values in the parentheses are of the ordinarily annealedmaterial.

What we claim is:

1. A process for reducing the thickness of a slab of rimmed steel to acold-rolled steel sheet having superior deep drawability, consistingessentially of the steps of hot-rolling the slab of rimmed steel,normalizing the hotrolled steel at a temperature ranging from the Atransformation point to 40 C. above the A transformation point for atime sufiicient to adjust the crystal grain size to 7.0 to 7.5 A.S.T.M.,cold-rolling the normalized steel at a reduction rate of from 40 to 80%,and annealing the cold-rolled steel for recrystallization.

2. A process for reducing the thickness of a slab otf rimmed steel to acold-rolled steel sheet having superior deep drawability, consistingessentially of steps of hotrolling the slab of rimmed steel,cold-rolling the hot-rolled steel at a reduction rate of less than 50%,normalizing the thus cold-rolled steel at a temperature ranging from theA transformation point to 40 C. above the A transformation point for atime sufiicient to adjust the crystal grain size to 7.0 to 7.5 A.S.T.M.,cold-rolling the normalized steel at a reduction rate of from 40 to 80%,and annealing the cold-rolled steel for recrystallization.

3. In a process for producing from a slab of rimmed steel cold-rolledsteel sheet having superior deep drawability, the steel having beenhot-rolled from the slab of rimmed steel and, when cold rolled inaddition to the hot rolling, having been cold-rolled at a reduction rateno greater than the steps consisting essentially of normalizing therolled steel, said normalizing step being carried out at a temperatureranging from the A transformation point to 40 above the A transformationpoint for a time sufiicient to adjust the crystal grain size to 7.0 to7.5 A.S.T.M., cold-rolling the normalized steel at a reduction rate offrom 40 to and annealing the cold-rolled steel (for recrystallization.

References Cited by the Examiner UNITED STATES PATENTS 1,874,144- 8/1932Tytus 14812.4 2,381,435 8/1945 Burns et al. 148-12 OTHER REFERENCES TheMaking, Shaping and Treating of Steel, US. Steel, Seventh Edition, 1957,page 813.

DAVID L. RECK, Primary Examiner.

H. F. SAITO, W. B. NOLL, Assistant Examiners.

1. A PROCESS FOR REDUCING THE THICKNESSOF A SLAB OF RIMMED STEEL TO ACOOLD-ROLLED STEEL SHEET HAVING SUPERIOR DEEP DRAWABILITY, CONSISTINGESSENTIALLY OF THE STEPS OF HOT-ROLLING THE SLAB OF RIMMED STEEL,NORMALIZING THE HOTROLLED STEEL AT A TEMPERATURE RANGING FROM THE A3TRANSFORMATION POINT TO 40*C. ABOVE THE A3 TRANSFORMATION POINT FOR ATIME SUFFICIENT TO ADJUST THE CRYSTAL GRAIN SIZE TO 7.0 TO 7.5 A.S.T.M.,COLD-ROLLING THE NORMALIZED STEEL AT A REDUCTION RATE OF FROM 40 TO 80%,AND ANNEALING THE COLD-ROLLED STEEL FOR RECRYSTALLIZATION.